Process for the electrolytic deposition of metal layers

ABSTRACT

The invention relates to a method for the electrolytic deposition of metal coatings, in particular of copper coatings with certain physical-mechanical and optical properties and uniform coating thickness. According to known methods using soluble anodes and applying direct current, only uneven metal distribution can be attained on complex shaped workpieces. By using a pulse current or pulse voltage method, the problem of the coatings being of varying thickness at various places on the workpiece surfaces can indeed be reduced. However, the further problem of the geometric ratios being changed continuously during the depositing process by dissolving of the anodes is not resolved thus. This can be avoided by using insoluble anodes. In order to guarantee sufficient stability of the anodes and a bright coating even at those points on the workpiece surfaces, onto which the metal is deposited with high current density, it is essential to add compounds of an electrochemically reversible redox system to the depositing solution.

The invention relates to a method for the electrolytic deposition ofmetal coatings with uniform coating thickness, particularly of coppercoatings with certain physical-mechanical and optical characteristics.

BACKGROUND OF THE INVENTION

In order to achieve certain physical-mechanical properties in metalcoatings, which can be deposited electrolytically, certain additivecompounds must be added in small amounts to the deposition solution. Ofmain concern in this respect are organic materials, which have an effecton the bright finish, the levelling and the uniformity of the depositionon large surfaces, avoidance of so-called burnt-on particles, i.e.deposition of granular crystalline coatings and also the construction ofmetal coatings with high fracture elongation and tensile strength.

The disadvantage in this respect is that these materials generallydisintegrate during deposition, so that they have to be replenishedduring the operation. Admittedly, the observation of constant conditionsin production is mostly very difficult, since the materials themselvesare only present in very small concentrations in the depositionsolutions, and in addition a complicated mixture of several materials ofthis type is also most often required to achieve certain coatingproperties and finally during dissolution degraded products are formedalso, which have an effect on the metal coating properties. Therefore,an analytical survey of the additive compounds is not only verydifficult, but is generally not adequate either for completelydescribing the state of the deposition bath, with the result thatanalytical methods for controlling the bath may only be used in aqualified manner.

In addition there is a demand, in the coating of complex shapedworkpieces for example of circuit boards with very fine borings, forachieving as uniform a thickness of metal coating as possible on allpoints of the workpiece. It is possible, with appropriate depositionbaths with optimised composition, to enlarge the metal coating thicknesseven in places with a low current density. However, the named additivecompounds only influence the metal dispersion so slightly that theproblem was not solved by these optimising measures.

In particular, it was not possible with the named measures to achieveeven an adequately uniform distribution of metal coating thickness incomplex shaped workpieces, for example in circuit boards with very fineborings.

Various means of solving the problem have been suggested therefore inthe literature, however none of them have yet led to entirelysatisfactory solutions.

As a solution for the equalising of the metal distribution on thesurfaces of the workpieces which are to be coated, the use of insolubleanodes during metal deposition is suggested. Anodes of this type areknown from the German Patent document DD 215 589 B5 and in thepublication DD 261 613 A1. Furthermore methods of this type are alsodescribed in DE 43 44 387 A1. In these publications mention is made alsoof the addition of compounds of electrochemically reversible redoxsystems to the depositing solution, with which systems the addition ofmetal salts for completing the deposited metal ions should be avoided.

A periodic current reversal during electrolysis is suggested as afurther solution for equalising the coating thickness on the workpieces("Pulse Plating-Elektrolytische Metallabscheidung mit Pulsstrom", Ed.Jean-Claude Puippe and Frank Leaman, Eugen G. Leuze Verlag, Saulgau,Germany, 1986, p.26 and "Pulse Plating of Copper for Printed CircuitTechnology" M. R. Kalantary, D. R. Gabe, Metal finishing 1991 pp. 21 to27). However, adequate uniformity of the deposited metal coatings cannotbe achieved in this way on large and in addition complex shapedworkpieces.

Furthermore, in DE 27 39 427 C2 a method for uniform coating of profiledworkpieces, which have narrow recesses, is described. For this purpose,the recesses in the surface of the workpiece are treated veryintensively with electrolytic solution and, at the same time, anelectric cycle of pulses lasting from 1 μsec to 50 μsec withconsiderably larger breaks in between was applied to the workpiece. Thisprocedure is very costly however, since a targeted injection at theprofiles in workpiece surfaces is not possible at least for massproduction or it requires a very high industrial fitting cost.

In WO-89/07162 A1 an electrochemical method for depositing metals isdescribed, preferably for copper from a sulphuric acid copperelectrolyte with organic additive compounds for improving thephysical-mechanical properties of workpieces, for example circuitboards. For this purpose, alternating current with varying long cathodicand anodic pulses is used. Coatings are successfully deposited oncomplex shaped workpieces, such as for example circuit boards, with moreuniform coating thickness. Further indications for overcoming theproblem of avoiding varying coating thicknesses by altering thegeometric ratios in the electrolytic cell, for example by dissolving theanodes, are not offered.

When the experiments described there were repeated, an improvement inthe metal dispersion in the fine borings of circuit boards could only beachieved, in our own findings, if at the same time, the opticalappearance of the copper coating, deposited according to the methoddescribed in the publication, became worse. Furthermore, the ductilityof these coatings was so slight that, even when a coated circuit boardwith borings was immersed just once for ten seconds into a 288° C. hotsoldering bath, fractures in the copper coating, particularly at thetransition from the circuit board surface to the boring wall, could beseen.

In the essay "Hartverchromung mittels eines Gleichrichters mitpulsierenden Wellen und periodischer Umkehr der Polaritat" by C.Colombini in the Professional Paper Galvanotechnik, 1988, pp. 2869 to2871 a method is likewise described, in which the metal coatings aredeposited not by direct current but rather by pulsating alternatingcurrent. According to the author's proposal this serves for producingchrome coatings, which are more corrosion proof than traditionalcoatings. Admittedly, it states in this publication that chromecoatings, formed according to this method, are grey and do not shine,with the result that they must be polished subsequently to produce aglossy surface. Apart from the fact that a subsequent mechanicaltreatment of this type is very exacting and therefore very expensive, inmany cases this cannot be carried out at all, for example when thesurface spots, which are to be treated, are not accessible.

In the essay "Pulse Reverse Copper Plating for Printed Circuit Boards"by W. F. Hall et al., Proc. of the American Electrochemical Society,10^(th) Plating in the Electronic Industry Symposium, San Francisco,Calif., February 1983 it has been shown, furthermore, that coppercoatings, which have been deposited by means of a pulsing currentmethod, can be formed on circuit boards with a more uniform coatingthickness from depositing solutions with brighteners, as with coppercoatings which are deposited by direct current.

The copper coatings are matt, according to information in thepublication, partly even brown or orange and do not consist thus of purecopper. To this extent it is surprising that, according to the author'sdata, high ductility values, namely high fracture elongation values, andtensile strength values, can be achieved using the method indicated.However, no adequately precise data about the deposition conditions, as,for example, the bath composition, temperature of the bath or the anodesused are given.

In EP 0 356 516 A1 a device for depositing electroplated coatings isshown, with which the physical-mechanical properties of the coatings canbe improved, in the view of the inventor. For this purpose, theamplitude, shape and frequency of the currents flowing through theelectrolytic bath during deposition are automatically changed. It isalso stated that, by measuring and stabilising the current in theelectroplating bath during deposition of the electroplated coatings, thephysical-mechanical properties are likewise improved.

In EP 0 129 338 B1 a method for electrolytic treatment of the surface ofa metal rail using graphite electrodes is described, in which, by usingalternating current with asymmetrical positive and negative half-wavesduring the electrolytic treatment, the dissolution of the graphiteelectrodes used as anodes can be avoided, with the result that thedistribution of current in the graphite electrode no longer alters andconstant conditions can be maintained during electrolysis. Admittedly,no indications are given in this publication of how an improvement canbe achieved in the physical-mechanical properties of deposited metalcoatings and, at the same time, how to make the distribution of coatingthickness as uniform as possible over a long period of operation.

THE PRESENT INVENTION

In order to produce circuit boards simply and economically, it isnecessary to deposit copper coatings with very good mechanical-physicalproperties, in particular with a high fracture elongation and uniformbrightness even in the high current density region. Since increasinglyfine borings are contained in the circuit boards and, because of theincreasing integration of components on the circuit boards, even greaterdemands are put on the uniformity of the coating thickness on thesurface of the circuit boards, electrolytic deposition procedures forcopper must be found, where the demands mentioned can be met. With theknown methods, however, particularly after a long operational time adeposition of high-quality and uniformly thick metal layers, even onlarge surface substrates, which may also be complex in shape, is nolonger reliably possible with a depositing bath without costly cleaningprocedures or even with a new make-up. In particular, it is not possibleto attain the demands mentioned (with the known methods) even whileusing higher current densities. Since, as is normal in this case,granular crystalline metal deposits are obtained (burnt-on particles),the physical-mechanical as well as the optical properties of coatingsproduced in this way are unsatisfactory, with the result that for thatreason also there is need for an improvement in the known methods.

Hence the problem underlying the present invention is to avoid thedisadvantages of the methods according to the state of the art and tofind a simple and economic method for the electrolytic deposition ofmetal coatings, especially made of copper, the metal coatings, which aredeposited according to the method, having very good physical-mechanicaland optical properties, for example brightness, even in the places onthe workpiece surfaces on which the metal is deposited with high currentdensity, and having high fracture elongation even after a fairly longoperational period in a depositing bath and having metal coatingthickness which is almost the same in all places on the surface of thetreated item, including within fine borings. In addition, thephysical-mechanical properties of the coatings should accord with thehighest demands even when using high current densities, for example ofat least 6 A/dm² on the surface to be coated (amongst other things withregard to fracture elongation and tensile strength).

The problem is resolved by Patent claims 1 and 11. Preferred embodimentsof the invention are given in the Sub-claims.

SUMMARY OF INVENTION

The invention is a method for electrolytic deposition of finecrystalline metal coatings, by means of a pulse current or pulse voltagemethod on complex shaped workpieces as cathodes, by using inertinsoluble anodes coated with noble metals or oxides of noble metals andforming a deposition solution that contains ions of the metals to bedeposited and certain compounds.

It has been shown that it is possible to improve the distribution of thecoating thickness in deposited metal coatings, particularly in coppercoatings, on the surface of complex shaped workpieces and in the boringsin the workpieces by means of a pulse current or pulse voltage procedurewithout thus impairing the physical-mechanical properties of thedeposited metal coatings, in particular the uniform brightness and highfracture elongation. When dimensionally stable, insoluble anodes areused for the anodes, the metal distribution on the outsides of theworkpiece can also be maintained considerably more uniformly. If thedepositing solution finally contains, besides the ions of the depositedmetal and the additive compounds for controlling the physical-mechanicalproperties, also compounds of an electrochemically reversible redoxsystem, by means of whose oxidised form moreover the ions of thedeposited metal can be formed by dissolving corresponding metal parts,then flawless coatings can also be obtained optically and in thephysical-mechanical properties by using higher current densities (forexample over 6 A/dm²).

In particular, copper coatings with a uniform high brightness can bedeposited on circuit boards with fine borings, said copper coatings alsohaving the capacity to withstand repeated thermal shock treatment in asoldering bath (immerssion for 10 seconds in a 288° C. hot bath andcooling subsequently at room temperature respectively), withoutfractures forming in the copper coating. In this respect the degree ofmetal coating distribution in the borings and on the surface of thecircuit boards which can be achieved is very good. For that reason it ispossible to deposit smaller amounts of metal on the circuit boards thanwith known methods, since the required layer thicknesses in the boringscan be achieved more quickly.

In known procedures where insoluble anodes are used and direct currentis applied it has been observed that the physical-mechanical propertiesof the deposited metal coatings as well as the distribution of the metalcoating on the workpiece become worse when the depositing bath isoperated over a fairly long period of time and has hence become old.This disadvantageous effect is likewise not established when using theclaimed method.

BRIEF DESCRIPTIONS OF DRAWING FIGURES

FIG. 1 is a diagrammatic illustration of a typical pulse cycle.

FIG. 2 is a diagrammatic illustration of another typical pulse cycle.

FIG. 3 is a diagrammatic illustration of a preferred current/voltagepulse cycle.

FIG. 4 is a diagrammatic illustration of a typical arrangement fortreating workpieces by the immersion method, in accordance with thisinvention.

DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS

By means of the measures according to the invention it is possible fororganic additive compounds to be used in the depositing solutions inorder to produce metal coatings with the predetermined propertieswithout the latter being degraded to a significant extent. The use ofspecific mixtures of additive compounds is not necessary. Furthermore, ahigh cathodic current density is attained. By means of this, an economicmethod is made possible, since the workpieces, which are to be treated,must remain in the unit for only a relatively short time in order to becoated with a metal coating of the predetermined thickness. A longlife-span of the dimensionally stable, insoluble anodes can be attained,since only extremely small amounts of aggressive reaction products ariseon the anodes.

In the pulse current method, the current is set galvanostaticallybetween the workpieces, which are polarised as cathodes, and the anodesand modulated temporally by appropriate means. In the pulse voltagemethod a voltage is set potentiostatically between the workpieces andthe anodes and the voltage is modulated temporally, so that atime-varying current is set.

By means of the pulse current or pulse voltage method, varying voltagesare applied to the workpiece or varying currents are set between theworkpiece and the dimensionally stable, insoluble alternativeelectrodes. For example, a cycle of pulse currents with anodic andcathodic current pulses is repeated periodically on the workpieces andif necessary with resting periods in between with the current strengthzero. With corresponding adjustment of a current pulse cycle, thecurrent pulse series mentioned is then set.

In a preferred embodiment, the current of the anodic current pulses isset to at least the same value as the current of the cathodic currentpulses in the workpieces, preferably to a value which is two to threetimes as high as the value of the cathodic current pulses.

The duration of an anodic current pulse in the workpieces is set forexample between 0.1 milliseconds (msec) and 1 second. The anodic pulselengths are preferably of 0.3 to 10 milliseconds. In all, the quantityof charge for depositing the metal must be greater than that leading tothe anodic redissolution of the metal from the workpiece.

Typical pulse cycles are represented in FIGS. 1 and 2. In FIG. 1firstly, a cathodic current pulse with a duration of 9.5 milliseconds isapplied to the workpiece. This is followed immediately by an anodiccurrent pulse, which has a peak value of roughly twice to three times asmuch. This double pulse is repeated periodically with a frequency of 100Hertz.

In FIG. 2 a pulse cycle is shown, which gives improved results. Thefirst cathodic phase follows a 5 millisecond rest phase, in which thecurrent is zero. The subsequent cathodic current pulse lasts for 5milliseconds. After that, there follows yet another 5 millisecond restand subsequently once again a 5 millisecond cathodic current pulse. Onlyafter that follows a 1 millisecond long anodic current pulse, the peaklevel of which is only a little higher than that of the cathodic currentpulses.

A preferred current/voltage pulse cycle is given in FIG. 3. In thiscycle a further substantial improvement in the metal distribution isachieved. The distribution of coating thickness, particularly betweennarrow recesses, for example in thin borings in circuit boards and theremaining surface regions is markedly improved in this way. Thefollowing typical values are set for the individual current/voltagephases:

1. Cathodic pulse: approx. 14 msec, approx. 6 A/dm²

2. Rest phase: approx. 1 msec,≈0 A/dm²

3. Anodic pulse: approx. 0.7 msec, approx. 15 A/dm²

With suitable equipment for producing the described pulse cycles,several cathodic and/or anodic current pulses with varying peaks ofcurrent can also be set.

In a particular embodiment form, either an anodic current pulse or aresting period with current strength zero is set between two cathodiccurrent pulses in the workpieces alternatingly.

No soluble anodes from the metal to be deposited are used as anodes,instead dimensionally stable, insoluble anodes are used. By usingdimensionally stable, insoluble anodes a constant spacing can be setover time between the anodes and the workpieces. The anodes can beaccommodated without problem in their geometrical shape to theelectroplated item and, in contrast to soluble anodes, hardly changetheir geometric dimensions. Hence the spacing affecting the distributionof coating thickness on the surface of the electroplated item remainsconstant between the anode and the cathode.

In order to produce insoluble anodes, materials inert with respect tothe electrolyte are used during electrolytic deposition such as, forexample, stainless steel and lead. Preferably, anodes are used however,which contain titanium or tantalum as the basis material which ispreferably coated with noble metals or oxides of the noble metals. As acoating, platinum, iridium or ruthenium, for example are used and alsothe oxides or mixed oxides of these metals. Besides platinum, iridiumand ruthenium, also rhodium, palladium, osmium, silver and gold or theiroxides and mixed oxides may, in principle be used for coating. Anespecially high resistance capacity relative to the electrolyticconditions could be observed for example in a titanium anode with aniridium oxide surface which was bombarded with fine particles, forexample sphere-shaped bodies, and hence coated non-porously.

The amount of aggressive reaction products which arise on the anode isaffected by the anodic current density. It was established that theirrate of formation is very small beneath an anodic current density of 2A/dm². Hence, the electrochemical effective anode surface selected mustbe as large as possible. In order to achieve effective anode surfaces,which are as large as possible, nevertheless within small spatiallimitations, perforated anodes, anode meshes or expanded metal with anappropriate coating are used. In this way it is guaranteed as well thatthe anode surfaces can be heavily exposed to an intensive through-flowof the perforated anode materials. In the first place, the diffusioncoating thickness on the anode is thus reduced, so that only a slightover-voltage arises on the anode, and on the other hand, any reactionproducts present are quickly removed from the anode surface. Meshes andexpanded metal may also be used in several layers, so that the anodesurface is increased even more and the anodic current density can thusbe reduced even further. The anodic surfaces should furthermore be freeof pores, which extend down to the underlying material.

Since the metal ions, which were spent in the deposition from thedepositing solution, cannot be directly supplied subsequently by meansof dissolution by the anodes, said metal ions are replenished bychemical dissolution of corresponding metal parts. For this purpose,compounds of an electrochemically reversible redox system are added tothe deposition solution, the oxidised form of said system forming themetal ions in a redox reaction from the metal parts.

In order to replenish the metal ions, which are spent by deposition, ametal ion generator is thus used, in which parts of the metal to bedeposited are contained. In order to regenerate the depositing solution,which has been depleted by consumption of metal ions, said depositingsolution is led past the anodes, causing the oxidising compounds of theredox system to be formed from the reduced form. Subsequently, thesolution is led through the metal ion generator, causing the oxidisingcompounds to react with the metal parts while constructing metal ions.Simultaneously, the oxidising compounds of the redox system areconverted into the reduced form. Because of the formation of metal ions,the entire concentration of the metal ions, which are contained in thedepositing solution, is kept constant. From the metal ion generator, thedepositing solution goes back again in to the electrolytic chamber,which is in contact with the cathodes and anodes.

Iron(II) and iron(III) compounds are used as an electrochemicallyreversible redox system. Equally appropriate are the redox systems ofthe following elements: titanium, cerium, vanadium, manganese andchrome. They can be added to the copper depositing solution for examplein the form of titanyl-sulphuric acid, cerium(IV)sulphate, sodiummetavanadate, manganese(II)sulphate or sodium chromate. Combined systemscan be advantageous for special applications.

After a short while the active Fe²⁺ /Fe³⁺ redox system is formed fromthe iron(II)sulphate-heptahydrate which is added to the depositingsolution. Said system is exceedingly suitable for aqueous, acidic copperbaths. Other water-soluble iron salts, in particulariron(II)sulphate-nonahydrate can also be used, as long as they do notcontain any biologically non-degradable (hard) complexing agents whichcan present problems in the waste water disposal (for example ferricammonium alum). The use of iron compounds with anions, which lead toundesired secondary reactions in the copper depositing solution, such asfor example chloride or nitrate, must likewise not be used.

The concentrations of compounds in the redox system must be arranged insuch a way that, by dissolving the metal parts, a constant concentrationof metal ions can be maintained in the depositing solution. Preferably,a concentration of at least 10 g of iron ions/liter of depositingsolution is set. It is guaranteed thus, that the insoluble anodes, whichare coated with noble metals or oxides of the noble metals, are notdamaged. Furthermore, the problem is also avoided in which the noblemetals or its oxides, which are eroded by the inert carrier materials ofthe anode, are not deposited on the copper pieces which are situated ifnecessary in a suitable separate container and which are dissolved bythe effects of the iron(III) ions, in order to keep the copper ionsconcentration constant in the depositing solution. Moreover, theformation of granular crystalline metal coatings is hence also avoidedin the high current density region (burnt-on particles).

The basic composition of a copper bath can vary within relatively widelimits when using the method according to the invention. In general, anaqueous solution of the following composition is used (all values ing/liter depositing solution):

    ______________________________________                                        copper sulphate (CuSO.sub.4 · 5H.sub.2 O)                                                 20-250                                                   preferably           80-140 or 180-220                                        sulphuric acid, conc.                                                                              50-350                                                   preferably           180-280 or 50-90                                         iron(II)sulphate(FeSO.sub.4 · 7 H.sub.2 O)                                                1-120                                                    preferably           20-80                                                    chloride ions (added 0.01-0.18                                                for example as NaCl)                                                          preferably           0.03-0.10.                                               ______________________________________                                    

In place of copper sulphate other copper salts may be used at least inpart. The sulphuric acid can also be replaced in part or completely byfluoroboric acid, methane sulphonic acid or other acids. The chlorideions are added as alkali chloride, for example sodium chloride, or inthe form of hydrochloric acid, analytically pure. The addition of sodiumchloride may be omitted completely or in part, if halogenide ions arealready contained in the additives.

Besides copper, other metals such as, for example, nickel or its alloyscan also be deposited in principle using the method according to theinvention.

In addition, conventional brighteners, levelers, wetting agents andother additives may be added to the depositing solution. In order toobtain bright copper deposits with predetermined physical/mechanicalproperties, at least one water-soluble sulphur compound and anoxygen-containing, highly-molecular compound are added. Additivecompounds such as nitrogenous sulphur compounds, polymeric nitrogencompounds and/or polymeric phenazonium compounds may also be used.

The additive compounds are contained in the depositing solution withinthe following concentration ranges (all values again in g/liter ofdepositing solution).

    ______________________________________                                        Conventional oxygen-containing,                                                                       0.005-20                                              high-molecular compounds                                                      preferably              0.01-5                                                conventional water-soluble                                                                            0.0005-0.4                                            organic sulphur compounds                                                     preferably              0.001-0.15                                            ______________________________________                                    

Some oxygen-containing, high-molecular compounds are listed in Table1.

Table 1

Oxygen-Containing, High-Molecular Compounds

carboxymethylcellulose

nonylphenolpolyglycol ether

octandiolbis-(polyalkylene glycolether)

octanolpolyalkylene glycolether

oleic acidpolyglycol ester

polyethylenepropylene glycol

polyethylene glycol

polyethylene glycoldimethylether

polyoxypropylene glycol

polypropylene glycol

polyvinylalcohol

stearic acidpolyglycol ester

stearyl alcoholpolyglycol ether

β-naphtholpolyglycol ether

In Table 2, there are various sulphur compounds with appropriatefunctional groups for producing water-solubility.

Table 2

Sulphur Compounds

3-(benzthiazolyl-2-thio)-propylsulphonic acid, sodium salt

3-mercaptopropane-1-sulphonic acid, sodium salt

ethylendithiodipropylsulphonic acid, sodium salt

bis-(p-sulfophenyl)-disulphide, disodium salt

bis-(ω-sulfobutyl)-disulphide, disodium salt

bis-(ω-sulfohydroxypropyl)-disulphide, disodium salt

bis-(ω-sulfopropyl)-disulphide, disodium salt

bis-(ω-sulfopropyl)-sulphide, disodium salt

methyl-(ω-sulfopropyl)-disulphide, disodium salt

methyl-(ω-sulfopropyl)-trisulphide, disodium salt

O-ethyl-dithiocarbonic acid-S-(ω-sulfopropyl)-ester, potassium saltthioglycolic acid

thiophosphoric acid-O-ethyl-bis-(ω-sulfopropyl)-ester, disodium salt

thiophosphoric acid-tris-(ω-sulfopropyl)-ester, trisodium salt.

Thiourea derivatives and/or polymer phenazonium compounds and/or polymernitrogen compounds are used as additive compounds in the followingconcentrations (all values in g/liter of depositing solutions):

    ______________________________________                                                      0.0001-0.50                                                            preferably                                                                           0.0005-0.04                                                     ______________________________________                                    

In order to make-up the depositing solution, the additive compounds areadded to the basic composition which is presented here. The conditionsof the copper deposition are given in the following:

    ______________________________________                                        pH value:             <1                                                      temperature:          15° C.-50° C.                             preferably            25° C.-40° C.                             cathodic current density:                                                                           0.5-12 A/dm.sup.2                                       preferably            3-7 A/dm.sup.2                                          ______________________________________                                    

By injecting air into the electrolytic chamber, the depositing solutionis stirred. By additional injection of the anodes and/or of the cathodewith air, the convection is increased in the region of the respectivesurfaces. Hence, the transport of materials in the vicinity of thecathode or the anode is optimised, with the result that greater currentdensities can be achieved. If applicable, aggressive oxidation means,occurring in small amounts, such as for example, oxygen and chlorine,are thus removed from the anodes. Moving the anodes and cathodes alsoimproves the transport of materials to the respective surfaces. In thisway, deposition which is constant and which has controlled diffusion isachieved. The movements can be achieved horizontally, vertically, inuniformly lateral movement and/or by means of vibration. A combinationwith injection of air is especially effective.

In an arrangement with a metal depositing solution, which is suitablefor carrying out the method according to the invention, there is/are

a. at least one first container for receiving a metal depositingsolution,

b. furthermore, metallic workpieces to be coated electrolytically withmetal and which are brought into contact with the depositing solution,

c. in addition, electrodes, which are arranged at a spacing relative tothe workpieces and which can be brought into contact with the depositingsolution as anodes, made from a material which does not dissolve bymeans of anodic reaction during metal deposition,

d. in addition, a voltage or current supply unit which is connectableelectrically with the electrodes and the workpieces and which isdesigned in such a way that the electrodes and the workpieces can beprovided with alternating voltage or current,

e. in addition at least one second container (copper ion generator) forreceiving pieces of the metal which is deposited on the workpieces, thesecond container being connected to the first container for transportingfluid in such a way that the depositing solution from the firstcontainer can be conveyed into the second and from there back to thefirst,

f. finally devices for conveying the depositing solution, for examplepumps, from the first container to the second and from there back to thefirst.

The metal depositing solution is then contained in the first containerand contains ions of the metal, which is to be deposited on theworkpieces, and compounds of an electrochemically reversible redoxsystem.

In the first embodiment (immersion method) the workpieces arealternatively arranged in the first container. In a second embodiment,the workpieces and the anodes can also be arranged outwith thecontainer. In this situation, there are devices provided for conveyingthe depositing solution from the first container to the workpieces, forexample pumps, in order to bring the workpieces and the anodes intocontinual contact with the depositing solution. This arrangement is usedin a horizontal conveyorized unit, as it can be used for treatingcircuit boards.

Normally coating units are used, in which the circuit boards are sunk ina vertical position into a container, containing the depositing solutionand are situated in this position opposite the dimensionally stableinsoluble anodes which are arranged on both sides. The anodes can beseparated by diaphragms from the catholytic chamber, in which thecircuit boards are situated. Appropriate as diaphragms are for examplewoven polypropylene or membranes with a metal ion and anionpermeability, such as for example Nafion membranes (from the company DuPont de Nemours Inc., Wilmington, Del., U.S.A.). In this arrangement,the depositing solution is pumped firstly to the circuit boards, whichare polarised as cathodes, and lead from there to the anodes. Thecathode and anode surfaces are injected by spray nozzles assemblies.This unit comprises besides the electrolytic cell with the container,the copper ion generator, into which the depositing solution, comingfrom the anodes, proceeds. There the depositing solution is enrichedagain with copper ions.

A typical arrangement, which is suitable for treating workpieces by theimmersion method, is represented schematically in FIG. 4. In thecontainer 1 the depositing solution 2, which contains compounds of theelectrochemically reversible redox system, for example iron(II) andiron(III) ions, is situated. The depositing solution can be used forexample for copper-plating and contains then the previously mentionedcomponents.

The workpieces 3, for example circuit boards, and the anodes 4, forexample titanium anodes coated with iridium oxide, are immersed into thedepositing solution. The workpieces and the anodes are connected to thecurrent source 5. Instead of regulating the current with the currentsource, there can also be a voltage arrangement, with which the voltagebetween the workpieces and the anodes is regulated. The depositingsolution is directed continuously to a second container 6 by means oftransporting equipment, which is not shown, for example pumps.

In this separate receptacle, namely the metal ion generator, which thedepositing solution flows through, the metal in the depositing solutionis replenished. In the metal ion generator, are situated, in the case ofcopper coating, metallic copper parts for example in the form of pieces,balls or pellets. The copper parts dissolve under the effects of theoxidised form of the redox compounds into copper ions. By dissolving thecopper parts, the oxidised form of the redox system is converted intothe reduced form. The solution which is enriched with the copper ionsand the reduced form is directed back again to the first container bymeans of pumps which are not shown. The metallic copper used forregeneration does not need to contain phosphorus, but phosphorus alsodoes not cause interference. In the traditional use of soluble copperanodes, the composition of the anode material is, on the other hand,greatly important. In this situation, the copper anodes must containapprox. 0.05% by wt phosphorus. Materials of this type are expensive andthe phosphorus supplement causes residues in the electrolytic cell,which have to be removed by additional filtering.

In the circulation of the depositing solution, filters can also be addedfor separating mechanical and/or chemical residues. However incomparison to electrolytic cells with soluble anodes, there is lessrequirement, because the anode sediment, which arises from the additionof phoshorous, does not occur.

In the other preferred embodiment the circuit boards are transportedthrough a conveyorized unit in a horizontal position and with ahorizontal direction of movement. In the process depositing solution isinjected continuously from below and/or from above onto the circuitboards by means of splash nozzles or flood pipes. The anodes arearranged at a spacing relative to the circuit boards and are brought, inthe same way as the circuit boards, into contact with the depositingsolution by means of a suitable device. The circuit boards makeelectrical contact on the side and move on a plane, which is arrangedbetween the anode planes, right through the unit. If necessary, thedepositing solution can be suctioned off again after penetrating throughborings in the circuit boards by means of devices arranged on the sideof the circuit board opposite the nozzles. The circuit boards makeelectrical contact via clamps. The transport speed in the unit is 0.01to 2.5 cm/sec, preferably 0.2 to 1.7 cm/sec. The circuit boards aretransported by means of rollers or plates.

Using the method according to the invention, circuit boards,particularly with copper coatings, can be coated electrolytically on thesurfaces and on the surface areas of the borings, which have beenalready plated thinly with copper.

The following examples serve to explain the invention further:

EXAMPLE 1 COMPARATIVE EXAMPLE

In an electrolytic cell, which is provided with soluble copper anodescontaining phosphorous, an aqueous copper bath was used with thefollowing composition:

    ______________________________________                                        copper sulphate (CuSO.sub.4 · 5 H.sub.2 O)                                                  80     g/liter                                         sulphuric acid, conc.  180    g/liter,                                        iron-(II)-sulphate (FeSO.sub.4 · 7H.sub.2 O)                                                35     g/liter,                                        sodium chloride        0.08   g/liter                                         ______________________________________                                    

and the following brighttening additive compounds:

    ______________________________________                                        polypropylene glycol    1.5 g/liter,                                          3-mercaptopropane-1-sulphonic acid,                                                                   0.006 g/liter,                                        sodium salt                                                                   N-acetyl thiourea       0.001 g/liter.                                        ______________________________________                                    

At an electrolytic temperature of 30° C., copper was deposited with acurrent density of 4 A/dm² on to a circuit board provided with a thincopper laminate on the surfaces and a thin copper layer in the borings,said circuit board, having a thickness of 1.6 mm and borings of 0.4 mm,was provided with scores in the copper laminate caused by scratching,for judging the degree of leveling of the deposited copper coating.

A highly bright copper coating was obtained. The metal dispersion(coating thickness in the borings×100/coating thickness in the circuitboard surface), was however only 55%. The fracture elongation of acopper foil deposited from the solution was 21% (measured with thedishing test method with the Ductensiomat according to the DIN-ISOmethod 8401, described in R. Schulz, D. Nitsche, N. Kanani in Jahrbuchder Oberflachentechnik (Yearbook of surface technology), 1992, p46 ff.

EXAMPLE 2 COMPARATIVE EXAMPLE

With the electrolytic solution used in Example 1, a copper coating wasdeposited by means of a pulse current procedure. The pulse current cycleaccording to FIG. 1 contained the following current pulses:

Current, cathodic: current density 4 A/dm², duration 10 msec

Current, anodic: current density 8 A/dm² ; duration 0.5 msec

Relative to the result from Example 1, the metal dispersion improvedfrom 55% to 75%. However, no usable copper coatings could be obtained,since their appearance was unacceptable. The copper coating was simplymatt. In addition the fracture elongation of the copper foil, which wasdeposited under these conditions, deteriorated from 21% to 14%.

EXAMPLE 3 COMPARATIVE EXAMPLE

Example 1 was repeated with direct current. In the place of solublecopper anodes containing phosphorus, a titanium expanded metal, whichwas coated with mixed oxides, was used as a dimensionally stable,insoluble anode.

The deposited copper coatings were at first uniformly bright. Thephysical-mechanical properties were also satisfactory. However the metaldispersion values, which lay below the values given in Example 1, weremeasured. After a fairly long operation of the depositing bath, theappearance and the fracture elongation of the coatings deteriorated. Atthe same time, it was established that the mixed oxide coating of thetitanium anode was eroded. This led to the over-voltage in the anoderising greatly.

EXAMPLE 4

Using the electrolytic solution given in Example 1, a circuit board,which was provided with copper laminate, was again copper-platedelectrolytically. However, soluble copper anodes were not used, insteada dimensionally stable, insoluble anode was used. A titanium expandedmetal which was coated with mixed oxides served as anode. In addition, aconcentration of iron(II)sulphate(FESo₄.7H₂ O) of 75 g/liter was used inthe depositing solution. In order to regenerate the copper ions, thedepositing solution was pumped from the treatment container into aseparate container, which was filled with copper pieces. By oxidisingthe copper with iron(III) ions, which were acting as oxidisation mediumand which were being formed continuosly on the anodes by oxidising theiron(II) ions, the copper pieces were successively dissolved and copperions were formed. The depositing solution which was enriched with thecopper ions proceeded from this container back into the treatmentcontainer.

By using the pulse current cycle given in Example 2, a uniformly highlybright copper coating could be obtained on the circuit board. The metaldispersion did not deteriorate relative to this example and accordinglyhad considerably better values than according to Example 1.

After copper had been deposited from the solution after a protractedperiod of time (electrical charge flow per solution volume: 10Ahr/liter) the fracture elongation, calculated according to the dishingtest method, from this aged solution, and of a copper foil, which wasdeposited under the previously mentioned conditions, came to 20% andthus lay in the range of values conveyed in Example 1. The insolubleanodes show no signs of impairment.

The copper coating deposited on the circuit board surfaces and in theborings withstood a thermal shock test, without fractures appearing inthe copper coating, in particular, not in the transitions from thecircuit board surface to the borings either. For this purpose, thecircuit board was submerged twice in a 288° C. hot soldering bath andcooled between times at room temperature.

EXAMPLE 5 COMPARATIVE EXAMPLE

In a conveyorized unit serving to treat circuit boards horizontally,copper laminated circuit boards which were provided with a thin coppercoating in the borings, were copper-plated in an electrolytic solutionby means of direct current. Copper anodes containing phosphorous wereused as anodes. The electrolytic solution had the following composition:

    ______________________________________                                        copper sulphate (CuSO.sub.4 · 5H.sub.2 O)                                                    80 g/liter,                                           sulphuric acid, conc.   200 g/liter,                                          iron-(III)-sulphate (Fe.sub.2 (SO.sub.4).sub.3 · 9H.sub.2                                    35 g/liter,                                           sodium chloride         0.06 g/liter                                          with the following brightness-producing additive compounds:                   polyethylene glycol     1.0 g/liter,                                          3-(benzthiazolyl-2-thio)-propyl-                                              sulphonic acid, sodium salt                                                                           0.01 g/liter,                                         acetamide               0.05 g/liter.                                         ______________________________________                                    

At an electrolytic temperature of 34° C. and with a current density of 6A/dm², a bright copper coating was obtained on the laminate which wasprovided in advance with scores caused by scratching. The circuit boardwas subjected five times to a thermal shock test in a soldering bath. Nofractures appeared in the copper coating. The metal dispersion inborings with a diameter of 0.6 mm was 62%.

EXAMPLE 6 COMPARATIVE EXAMPLE

The experiment of Example 5 was repeated. In the first place, a pulsecurrent method according to FIG. 1 was used instead of direct currentwithin the following parameters.

Current, cathodic: current density 6 A/dm², duration 10 msec

Current, anodic: current density 10 A/dm², duration 0.5 msec

Relative to Example 4, a considerably improved metal dispersion wasachieved. It came to 85% in the 0.6 mm large borings. Certainly theappearance of the deposited copper coating had deteriorated noticeably.The copper coating was not uniform and matt spots had appeared. Duringthe thermal shock test, using the presently described conditions,fractures appeared on the copper coating after being submerged fivetimes in a soldering bath.

EXAMPLE 7

Under the conditions given in Example 4, circuit boards were coated withcopper by using a titanium expanded metal anode, which was coated withplatinum, instead of soluble copper anodes. In place of direct current,a pulse current method, within the parameters given in Example 6 wasapplied. In addition, the content of iron(II)sulphate(FeSO₄.7H₂ O) inthe depositing solution was raised to 80 g/liter.

The deposited copper coating was uniformly highly bright and thereforehad a considerably better appearance than the circuit board producedaccording to the method of Example 5. The circuit board was once againsubjected five times to a thermal shock test in a 288° C. hot solderingbath by submerging and cooling between times at room temperature. Nofractures could be observed in the copper coating. In addition, themetal dispersion values improved relative to Example 6. Values above 85%were measured. The insoluble anodes were also stable for a fairly longtime.

What is claimed is:
 1. Method for the electrolytic deposition of finecrystalline metal coatings with uniform distribution of layerthicknesses and uniform brightness, high fracture elongation and tensilestrength even in places of high current density, by means of a pulsecurrent method comprising a duration of cathodic, or cathodic andanodic, pulses of current or voltage, comprising the steps of:a)applying the pulse current on workpieces polarized as cathodes and b)using inert, dimensionally-stable insoluble anodes coated with noblemetals or oxides of noble metals c) from a deposition solutioncontainingc1) ions of the metal to be deposited, c2) additive compoundsfor controlling the brightness, the fracture elongation and the tensilestrength, and c3) compounds of at least one electrochemically reversibleredox system, by means of the oxidised form of which the ions of themetal to be deposited are formed by at least partial dissolution ofpieces of said metal.
 2. Method according to claim 1, where the pulsecurrent method comprises an adjustable current pulse sequence comprisinganodic current pulses and cathodic current pulses on the workpiece,where said adjustable current pulse sequence is periodically repeated.3. Method according to claim 2, characterised in that the current of theanodic current pulses is set at twice or thrice the height of thecurrent of the cathodic current pulses.
 4. Method according to claim 2,characterised in that the duration of an anodic current pulse on theworkpieces is set from 0.3 milliseconds to 10 milliseconds.
 5. Themethod of claim 2, including intermediate rest phases with zero currentdensity between anodic current pulses and cathodic current pulses. 6.Method according to claim 1 characterised in that, between two cathodiccurrent pulses at the workpieces alternately either one anodic currentpulse or a rest phase with the current density zero is set, or acombination of an anodic current pulse and a rest phase.
 7. Methodaccording to one of claims 1-2, characterised in that titanium expandedmetal layered with iridiumoxide and irradiated by means of fineparticles is used as an anode.
 8. Method according to one of claims 1-2,characterised in that iron-(II)- and iron-(III)-compounds are used as anelectrochemically reversible redox system.
 9. Method according to claim8, characterised in that an iron ion concentration of at least 10g/liter is set in the deposition solution.
 10. Method according to oneof claims 1-2, for electrolytic deposition of copper coatings onsurfaces and peripheral surfaces of bores of circuit boards.